Due to the critical importance of maintaining proper inflation pressures in the pneumatic tires of aircraft, it is the practice in the industry to provide each tire with its own pressure gage. Such gages are typically mounted in a well formed in the metal wheel which supports the pneumatic tire. In this manner, ground maintenance crews are provided with an accurate indication of the air pressure in each tire when performing routine service on the aircraft, and additionally can quickly and easily determine when the proper inflation pressure has been obtained in instances where it is necessary to inflate the tire.
The most common type of gage utilized for this purpose employs a coiled bourdon tube pressure sensor in combination with a conventional indicator dial readout. A sealed gage case surrounding the bourdon tube is constructed to withstand pressures in excess of the normal tire pressure in order to prevent loss of tire pressure in the event of a rupture or leak in the bourdon tube. The gage is normally provided with integral fill structure so that both gaging and filling operations can be accomplished via the same entry through the wheel to the interior of the tire.
If the bourdon tube in gages of this type should rupture, the tire pressure is maintained due to the sealed construction of the gage case as mentioned hereinabove. In such instances, maintenance crews are appraised of the gage failure because the indicator dial will show greatly reduced or no pressure even though the tire is fully inflated; the faulty gage is then simply replaced with a new one. However, in the much more common instance where the bourdon tube develops a relatively slow leak, the tire pressure will still be maintained at approximately the same level, but there may be no immediate indication that the gage is faulty. Under these circumstances, the gage may indicate a low-pressure reading when, in fact, the tire is properly inflated, so that as a consequence a ground maintenance worker may be misled into over inflating the tire. Perhaps of more critical concern, is the fact that the gage case can become partially pressurized for what may be an extended period of time, which, in turn, could result in blowout of the gage case, concurrent rupture of the bourdon tube, and appurtenant catastrophic loss of tire pressure.